One well known technique for coating surfaces with metal, ceramic or plastic is a thermal spray process. Generally, thermal spray processes typically involve the step of heat softening a heat fusible material such as a metal or ceramic followed by the step of propelling the heat softened material in particulate form against a surface that is to be coated with the heat softened material. The heat softened material and the surface form a bond. Thermal spraying can be accomplished with a flame spray gun that normally uses a combustion or plasma flame to effect melting of a powder. Other heating means such as electric arcs, resistant heaters or induction heaters can also be used, alone or in combination.
Thus, thermal spraying of metallic, ceramic or plastic powders on a surface is a useful process for imparting desired properties to a surface. For example, thermal sprayable molybdenum-iron alloy powders, as described in U.S. Pat. No. 5,292,382, are thermal sprayed on metal surfaces to improve wear resistance, such as sliding wear and/or fretting wear. Mineral coatings are thermal sprayed on plastic surfaces, as described in U.S. Pat. No. 4,388,373, to provide such surfaces with greater hardness and wear resistance. In other well known thermal spray processes, metallic and/or ceramic materials are thermal sprayed to make dimensional restorations to a worn surface, to provide corrosion resistance, or to improve its thermal conductivity. Also, plastics may be applied to a metal surface with a thermal spraying process to provide a metal surface with properties such as improved release, a low coefficient of friction, and non-wetting.
Metal or ceramic thermal sprayable powders typically have high melting points (for metals, around 2600.degree. F., and for ceramics, about 3000.degree. F.), and accordingly, they must be sprayed with high temperature gas streams, such as plasma sprays or acetylene gas, to transfer sufficient heat to melt them. In contrast to metallic or ceramic powders thermally sprayed plastic powders usually having lower melting points (from approximately 180.degree. F. to around 500.degree. F.), and therefore, such powders are usually thermal sprayed with low temperature gas streams, such as, hydrogen or natural gas, to prevent super heating and oxidation thereof.
In some coating applications, it is desirable to combine relatively low melting point plastic materials with relatively high melting point metallic or ceramic thermal sprayable materials to impart the particular properties of each material to a surface simultaneously as a duplex coating in a single application. However, the thermal spraying of such a combination usually provides poor and unacceptable end results because the melting points of the metal and plastic materials are incompatible. For example, the conditions needed to properly melt high melting point thermal spray powders, such as metal powder, will usually cause decomposition and loss of the plastic powder, thus preventing simultaneous deposition of a duplex coating containing both the high melting point metal with the low melting point plastic dispersed through out. On the other hand, if conditions are selected so as to properly melt the plastic powder but prevent decomposition thereof, then the high temperature metal powder will not melt, and accordingly, will not deposit on metal surfaces to form desired duplex coatings.
Surfaces requiring such duplex coatings to provide long wearing release or non-stick properties include rolls used in the paper producing or converting industry. In the past, coatings having metallic and plastic constituents have been applied to such rolls using a two step process wherein a roll is first thermal sprayed with a wear resistant and/or a corrosion resistant metal coating, such as a molybdenum-iron alloy, and then impregnated with a liquified plastic powder, such as PTFE. As those skilled in the art will appreciate, this prior art two step process is cumbersome, costly and difficult to accomplish on site as compared to the present invention. The prior art two step process has limited effectiveness even when applied over a good thermal spray coating, because such thermal spray coatings do not always have the required interconnected porosity to allow effective penetration of the plastic. Thus, the application of plastic to a roll with this two step prior art process usually only results in a surface effect that quickly wears away leaving the roll without the desired release properties imparted by the plastic. If adaptations in the prior art process are made so as to form a higher porosity metal that can absorb more plastic to provide a roll with durable release properties, the resulting matrix properties of the coating tend to be severely reduced. The reduction in matrix properties of such coatings can lead to surfaces that wear and/or corrode rapidly.
As those skilled in the art will appreciate, it is highly desirable to have the ability to control the amount of plastic and high temperature material deposited, as well as the ability to control the time and manner of application, on the surface of a substrate to provide the surface with a duplex coating having long wearing release or non-stick properties. However, this goal is nearly impossible with the prior art two step process.